As ring-shaped insert molded articles manufactured by injection molding in a state where a thermoset additive is applied to a joining surface of a ring-shaped metallic part as an insert to a plastic and then the insert and the plastic are placed in a metal mold, there have been suggested axial (axially opposed-type) magnetic encoders (for example, refer to Patent Documents 1 and 2, and Patent Document 3, FIG. 2) and radial (radial opposed-type) magnetic encoders (for example, refer to Patent Document 3, FIG. 9 and Patent Document 4), which are used in devices for detecting the rotation speed of a rotating body.
Such injection molding of magnetic encoders is performed in many cases such that a molten resin is injected into a metal mold from a disc gate placed at the radially inner side of the insert molded article for the viewpoint of the orientation of magnetic substance powder in the molten resin (refer to Patent Document 2, paragraphs [0017] to [0023], Patent Document 3, paragraph [0034], and Patent Document 4, the description in the prior art section).